I am not too familiar with smarty (yet), but if there are some more people that would contribute, we can try to enhance your script to even server a valid opds catalog and thus provide an even more convenient alternative to the resource intensive calibre-server. (There might be a couple of more people out there running a NAS box with an arm cpu that would actually love to use this? )

I did a quick perusal of the scripts earlier. Really, pretty much the only thing that needs to be changed are the smarty templates. Perhaps the index.php file, too, although from my initial review, it doesn't appear as if that's needed.